The present invention relates to brazing of metal parts and, in particular, to brazing filler metals useful for brazing gaps of thickness greater than about 100 .mu.m and width of several inches or more.
Brazing is a process for joining metal parts, often of dissimilar composition, to each other. Typically, a filler metal that has a melting point lower than that of the metal parts to be joined is interposed between the parts to form an assembly. The assembly is then heated to a temperature sufficient to melt the filler metal. Upon cooling, a strong, preferably corrosion resistant, joint is formed.
Conventional brazing filler materials exist in a wide variety of forms which are characteristic of metallic materials, namely: powders, pastes formed from powders, foils, strips and rods. Among these forms, strips and foils of brazing filler metals offer the most promise in the formation of uniformly brazed joints because of the relative ease of placement of the brazing filler metals into the assembly to be brazed.
Recently, a variety of alloys have been developed which can be cast into homogeneous, ductile, thin brazing foils by, for example, the casting process disclosed in U.S. Pat. No. 4,142,571. This casting process, known as planar flow casting, involves solidification of molten metal into a thin foil by casting onto a rapidly moving quenching surface. Alloys suitable for casting into such foils are disclosed, for example, in U.S. Pat. No. 4,405,391. However, homogeneous ductile brazing foil materials produced thusfar do not exceed about 90 .mu.m (.apprxeq.0.035 in) in thickness.
In many applications, however, the brazing gap thickness is greater than about 100 .mu.m and/or wider than about 250 mm (.apprxeq.10 in.) Accordingly, it has been necessary to individually place a plurality of the foils into the joint to be brazed, either in a stacked and/or side-by-side configuration. Unfortunately, problems are created in maintaining the layers in proper alignment with each other and, as a result, the use of a plurality of individual layers has not gained commercial acceptance.
It is known to consolidate a number of layers of at least 50% amorphous ribbon by the process disclosed in U.S. Pat. No. 4,529,457. Also, attempts have been made to use adhesives to consolidate multiple layers of these materials. In the former instance, however, copper-phosphorus and nickel-boron-silicon-base brazing foils become brittle on consolidation and, therefore, would have extremely limited use in brazing joints of complex shape. In the latter instance, use of adhesives produces the unacceptable result of unwanted residue or porosity in the brazed joint. As a result, non-uniform and, in many instances, unacceptably weak joints are produced.
There remains a need in the art for thick and/or exceptionally wide, flexible frazing foils which can accommodate brazing of large parts such as tail pipes of aircraft turbine engines.